1. Field of the Invention
The thermodynamic concept associated with electrochemical heat engines is that the free energy change of a reaction can be altered by changing the activities of the reactants or products. The following equation describes such activities or products, using accepted thermodynamic symbols: ##EQU1##
This equation describes the influences of variations of activity upon the values of .DELTA.G and of .epsilon. which is the voltage associated with an electrochemical cell reversibly carrying out the reaction. As noted, one way to change an activity is to change the pressure of a gas and such pressure changes are the basis of operation of the electrochemical heat engines of this invention. An electrochemical cell discharges and delivers a larger voltage than is required for the same cell to be recharged. To achieve this voltage gain, heat must be converted to work and that work must be used to assist the electrochemical charging reaction. A discharge reaction is selected in which one of the reactants exhibits a high vapor pressure at the cell temperature, and the discharge reaction is carried out at that high pressure. During recharging, the system pressure is sharply lowered by cooling a condenser which is in vapor contact with the cell but is far enough away from the cell so that the original cell temperature can be maintained. Under these conditions the volatile reactant is regenerated by electrochemical reaction but immediately moves as a vapor to the condenser where it collects as a condensed phase. The pumping action of the condenser assists the electrochemical recharging reaction. Prior to discharge the system pressure is again built up by heating the condenser. The introduction of this nonelectrochemical step in the cell cycle permits the discharge voltage to be greater than the recharging voltage, i.e., during discharge there is no action comparable to the pumping action occurring during recharge. In the cycle the heat of vaporization is supplied at the high temperature during the recharging reaction and heat of condensation is discharged and removed at the condenser temperature. Also heat equal to the difference in the .DELTA.G values for the discharge and for the recharging reactions is supplied at the high temperature. In terms of the heretofore stated equation, the pressures, activities, and changes of entropy and free energy are very different for the electrochemical reaction in its discharging and in its recharging directions.
The reversible work per cycle which can be recovered from any heat engine cycle is determined by the equation (T.sub.1 - T.sub.2)/T.sub.1, where T.sub.1 is the absolute temperature at which heat is supplied to the engine and T.sub.2 is the temperature at which heat is exhausted. An electrochemical engine's T.sub.1 and T.sub.2 are set by the vapor pressure of the working fluid being used, i.e., if the pressure and temperature are too high, then containment and/or heat transfer cannot be handled adequately. If the pressure and temperature are too low, the working fluid cannot be transferred fast enough from the cell to the condenser during recharging. The maximum pressure of an electrochemical cell is about 25 atm and the minimum pressure is about 10.sup.-4 atm. Although too low a condenser temperature is not necessarily harmful to engine operation, the Carnot limit associated with the low temperature will not be approached because the corresponding pressure cannot be maintained in a working engine if the pumping speed is too low. The .DELTA.G.degree. in the equation cancels between the forward and the reverse reactions and therefore the voltage difference between charge and discharge reflects the properties of the working fluid but is independent of the cell reaction selected. The cell reaction becomes very important if the cells are to be used as energy storage devices as well as electrochemical engines. There are advantages to generating comparatively large voltages per cell per cycle. The equation indicates that larger voltages are associated with high vs. low temperatures, monatomic vs. polyatomic gases, and univalent vs. polyvalent species in the ions generated from the working fluid of an electrochemical engine.
2. Prior Art
A. "Regenerative EMF Cells," Advances in Chemistry Series 64. This symposium reported on considerable work on chemical cells but did not recognize the versatility and applicability of electrochemical heat engines as opposed to simple thermally regenerative cells. This symposium reported only one limited and unpromising treatment of a cycle somewhat similar to the cycles described and claimed in this invention. The investigators disclosed a very poorly designed system utilizing lead-iodine, and therefore their cycle required excessive heating of the working fluid, iodine, while generating power at an exceedingly slow rate, and their anticipated efficiency fell far below the Carnot efficiency. Actually, if properly conceived, this type of electrochemical heat engine cycle can approach the Carnot ideal because no substance except the essential working fluid is heat cycled.
B. U.S. Pat. No. 3,458,356, J. T. Kummer et al., describes a method of generating electrical energy wherein a molten alkali metal at a first temperature and pressure in a first reaction zone is converted to cations with electron loss to electrical circuit in electrical communication with the alkali metal in said first zone, said cations pass through a cationic conductive barrier to mass fluid transfer to a second and significantly lower pressure and temperature in a second reaction zone and are reconverted to elemental form upon electron acceptance from said electric circuit within the said second zone. Thus, U.S. Pat. No. 3,458,356 describes a different class of electrochemical cycle wherein a pressure gradient created by a temperature gradient must be maintained across a solid ionic conductor. In practice (because of the limited number of such solid ionic conductors, largely of the beta-aluminum oxide class) such electrochemical cycles are almost entirely limited to sodium ion conduction. The electrochemical heat engine of this invention is operable with a large variety of liquid electrolytes and, as a consequence, has a much greater versatility as to heat ranges, heat sources, generator construction, etc.
3. Utility
Electrochemical heat engines provide a new method of generating electric power, and they can also be used for energy storage. Among numerous possible uses, when coupled with existing devices such as steam turbines they offer increased power generation efficiencies; for solar applications they offer both efficient energy conversion and storage of electric power, a combination which is not provided by other single devices. It is believed that such electrochemical engines can produce electricity at prices comparable to those from other energy conversion systems being proposed or in operation. The use of mixed alkali carbonates as opposed to pure carbonate as the molten electrolyte as will be described in the preferred embodiments greatly increases the feasible temperature ranges of the electrochemical heat engines of this invention. The extension of the temperature ranges makes for more versatile and more efficient heat engine operations and thus for the production of cheaper electricity while permitting use of heat sources at a wider variety of temperature ranges.